1. Introduction
The present invention relates to the control of electrical motors, and in particular, doubly-excited electrical motors with electronic commutation.
2. Field of the Invention
Generally speaking, such motors have a rotating field which may consist of a winding in which currents flow to produce a magnetic field, or permanent magnets to produce a magnetic field with a fixed value. The armature, or load current-carrying part is typically stationary and the magnitude, waveshape and phasing of the armature currents are controlled by an electronic commutator or controller.
The performance of these motors is described by the following relationships: EQU T=C.sub.1 .multidot.K.sub.T .multidot.COS.0..multidot.I EQU and, EQU E=C.sub.2 .multidot.K.sub.T .multidot.n.
where
T=motor output torque; PA0 K.sub.T =motor torque constant; PA0 c.sub.1,c.sub.2 =numerical scaling factor depending scaling used for alternative definitions of K.sub.T ; PA0 I=motor armature current; PA0 E=motor generated voltage (back e.m.f.); PA0 n=motor speed; and PA0 .0.=motor torque angle (defined below) PA0 means for continuously monitoring a parameter value of the motor stator winding electrical supply; PA0 means for continuously comparing the instantaneous parameter value of the stator winding electrical supply with the value of that parameter corresponding to full utilization of the motor's supply voltage; and PA0 means responsive to said comparison for continuously transmitting a signal to the torque-angle shift implementation means indicative of the amount of torque-angle shift to be implemented. PA0 means for continuously monitoring motor torque direction; and PA0 means for changing the polarity of the torque-angle shift when motor torque direction changes. PA0 continuously monitoring a parameter value of the motor stator winding electrical supply; and PA0 continuously comparing the instantaneous parameter value of the stator winding electrical supply with a value of that parameter corresponding to full utilization of the motor supply voltage; and PA0 implementing torque-angle shift at a level determined by said comparison.
From first principles, it is well established that K.sub.T is a function of motor construction and magnetic field strength from the field part of the motor. If the angle between field and armature fluxes in the motor is called .theta. then (90-.theta.) is referred to hereafter as the torque-angle, .0..
For poly phase AC machines .0. is also the angle between the fundamental harmonics of the armature current in a phase and phase-to-neutral generated voltage in that phase. This type of motor with permanent magnet field is commonly used in brushless servo systems. The electronic commutation in the motor controller controls the angle .0..
Usually, in brushless motor controllers the electronic commutation constrains the angle .0. to be 0.degree.. This gives COS.0. it's maximum value and gives maximum output torque per unit of input current. However, as the speed of the motor increases, so also does the back e.m.f. until a limiting point is reached when the sum of the back e.m.f. and the motor impedance voltage drop caused by the load current equals the supply voltage. When this happens the motor speed cannot increase beyond this maximum level known as the "base speed". This base speed varies with load torque.
In principle, it is possible to achieve speeds higher than the base speed by dynamically varying K.sub.T. This principle is known and it has been applied for wound field brush type DC motors by varying the field winding excitation current.
For brushless motors with permanent magnet excitation, varying K.sub.t by changing the field component magnetic field strength is not possible. However, it is possible to dynamically vary .0., which is under electronic control.
If .0. is increased from 0.degree. , the current in a motor winding will lead the back e.m.f. in that winding. This causes the motor impedance voltage drop to have a component which is in Antiphase with the back e.m.f. Thus, the sum of the motor impedance voltage drop and the motor back e.m.f. can be smaller than the motor back e.m.f. This allows speeds greater than base speed to be achieved even though the speed dependent back e.m.f. may be several times larger than the maximum available supply voltage.
Below base speed, clockwise torque is obtained when the armature flux is 90.degree. in a clockwise direction from the field flux whether the motor is turning clockwise (motoring) or counter-clockwise (generating). To obtain speeds higher than base speed the armature flux must be shifted further clockwise.
Below base speed, counter-clockwise torque is obtained when the armature flux is 90.degree. in an counter-clockwise direction from the field flux whether the motor is turning clockwise or counter-clockwise. To obtain speeds higher than base speed the armature flux must be shifted further counter-clockwise. Thus, the required direction of torque angle shift depends only on the direction of motor torque, not on the direction of motor velocity.
It is important to have maximum decelerating (generating) torque so that the speed of an inertial load can be reduced quickly. To obtain maximum decelerating torque at high speed, torque angle shift must also be implemented and the required direction is set by the torque direction. Heretofore this has not been done, with the result that there is a speed lag in response to applied decelerating torque.
Torque-angle shifting has been implemented recently for brushless DC motors using electronic commutation. For example, U.S. Pat. No. 4,490,661 discloses that it can be achieved by applying sinusoidal excitation currents to the stator windings, the currents having values selected in accordance with pre-recorded digital sine values selected in accordance with the motor speed, motor load and rotor position. To achieve this, an electronic programming function is used, which function is established by making assumptions regarding certain motor parameter values, for example:- motor winding resistance, winding inductance, torque constant K.sub.T and the DC supply voltage. To make these assumptions, it is necessary to allow for the least favorable conditions of the parameters and of ambient conditions. Because of this, the torque-angle will often be shifted unnecessarily, or by more than is required, resulting in lower output torque and efficiency. During deceleration from high speeds, torque angle shift is implemented in the opposite direction for a long enough time to cause a lag in deceleration as the supply voltage is being fully utilized.
The present invention is directed towards solving these problems by providing an improved control method and apparatus for limiting the amount of torque-angle shift to the minimum necessary in brushless motors.
In this specification, the term "stator winding electrical supply" covers a current or voltage signal used to determine the fraction of supply voltage applied to the stator windings of a brushless motor or alternatively the actual current or voltage in the stator windings.